The present invention generally relates to shimming magnetic resonance imaging (MRI) magnets and, more particularly, is concerned with a method for generating a ferromagnetic shim calibration file for use in shimming all MRI magnets.
Prior art methods exist that compute the magnetic field generated by a shim, a piece of ferromagnetic material, placed in a magnetic field. These methods assume that the external applied magnetic field is of sufficient strength to saturate the shim material. In these method, the solution of the magnetic field as a function of the weight of the shim is reduced to a straightforward calculation of the dipole moment of the shim in saturation. Thus, current shim calculation methods assumes that the shim material is in saturation.
More particularly, these methods compute the magnetic field for each shim at a certain location under the assumption that the shim is saturated (i.e. all magnetic domains in the shim material are aligned with the driving magnetic field). It also only takes into account the influence of the field generated by the shim in one direction (along the shim length). This method works satisfactory for cylindrical MRI systems, where the shim system is not influenced by nearby large masses of iron, such as the pole-face. It does not produce accurate accounts where such polefaces are present and the shims are located near such a pole-face.
In a magnet containing an iron or permanent-magnet pole-face, the shims are not necessarily saturated. To compute the influence of the magnetic field generated by a shim placed on or close to such a pole-face, a finite element analysis (FEA) solution is generally used. This is both time-consuming and inaccurate since a full three-dimensional solution is needed and the size of the shim is very small compared to the size of the magnet which causes difficulties in building a mesh for the finite-element code.
Thus, prior art methods assume saturation of the shims, and use an analytical approach to compute a shim calibration strength that is independent of shim shape. These methods are not useful in open geometry MRI magnets. Consequently, a need exists for an innovation which will overcome the problem with prior art methods without introducing new problems.
The present invention provides a ferromagnetic shim calibration file generating method designed to satisfy the aforementioned need. The method of the present invention generates a ferromagnetic shim calibration file which is accurate and useful in shimming all MRI magnets. The method makes use of FEA models to generate the local magnetic field at the shim location. The internal magnetization is used together with a shape factor correction algorithm to produce an accurate external magnetization.
In one embodiment of the present invention, a method for generating a ferromagnetic shim calibration file is provided which comprises the steps of performing a computation of an analytical Finite Element solution of an internal magnetic field for each location where a shim is assumed to be present in an imaging volume of a magnet, performing a computation of an equivalent magnetic dipole moment of a shim with a saturated magnetization using the analytical Finite Element solution of the internal magnetic field, and storing all computations for each shim in a shim calibration file. The method further comprises making a correction of the computation of the equivalent magnetic dipole moment of each shim to compensate for an aspect ratio for the shape of the shim and storing the correction for each shim in the shim calibration file. The method still further comprises making a correction of the computation of the equivalent magnetic dipole moment of each shim to compensate for a magnetic mirror effect in a pole-face of the magnet and storing the correction for each shim in the shim calibration file.